The present invention relates generally to automotive dampers or shock absorbers which receive mechanical shock. More particularly, the present invention relates to a unique hydraulic valve assembly which allows greater tunability of the shock absorber, especially in the mode of low hydraulic fluid flow.
Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur during driving. To absorb these unwanted vibrations, shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (suspension) of the automobile. A piston is located within a pressure tube of the shock absorber, with the piston being connected to the sprung portion of the automobile through a piston rod and the pressure tube being connected to the unsprung portion of the automobile. Because the piston is able, through valving, to limit the flow of damping fluid between opposite sides of the piston, when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the unwanted vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile. In a dual tube shock absorber, a fluid reservoir is defined between the pressure tube and the reservoir tube. A base valve can be located between the lower working chamber (the area below the piston) and the reservoir to limit the flow of fluid between the lower working chamber and the reservoir to produce a damping force which also counteracts the unwanted vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile. The greater the degree to which the flow of fluid within the shock absorber is restricted by the piston and/or the base valving, the greater the damping forces which are generated by the shock absorber. Thus, a highly restricted flow of fluid would produce a firm ride while a less restricted flow of fluid would produce a soft ride.
In selecting the amount of damping that a shock absorber is to provide, at least three vehicle performance characteristics are considered. These three characteristics are ride comfort, vehicle handling and road holding ability. Ride comfort is often a function of the spring constant of the main springs of the vehicle as well as the spring constant of the seat, tires and the damping coefficient of the shock absorber. For optimum ride comfort, a relatively low damping force or a soft ride is preferred.
Vehicle handling is related to the variation in the vehicle""s attitude (i.e., roll, pitch and yaw). For optimum vehicle handling, relatively large damping forces, or a firm ride, are required to avoid excessively rapid variations in the vehicle""s attitude during cornering, acceleration and deceleration.
Finally, road holding ability is generally a function of the amount of contact between the tires and the ground. To optimize road handling ability, large damping forces, or a firm ride, are required when driving on irregular surfaces to prevent loss of contact between the wheel and the ground for excessive periods of time.
Various types of shock absorbers have been developed to generate the desired damping forces in relation to the various vehicle performance characteristics. Shock absorbers have been developed to provide different damping characteristics depending on the speed at which the piston moves within the pressure tube. Because of the exponential relation between pressure drop and flow rate, it is a difficult task to obtain a damping force at relatively low piston velocities, particularly at velocities near zero. Low speed damping force is important to vehicle handling since most vehicle handling events are controlled by low speed vehicle body velocities.
Various prior art systems for tuning shock absorbers during low speed movement of the piston create a fixed low speed bleed orifice which provide a bleed passage which is always open across the piston. This bleed orifice can be created by utilizing orifice notches positioned either on the flexible disc adjacent to the sealing land or utilizing orifice notches directly in the sealing land itself. In order to obtain the low speed control utilizing these open orifice notches, the orifice notches have to be small enough to create a restriction at relatively low velocities. When this is accomplished, the low speed fluid circuit of the valving system will operate over a very small range of velocity. Therefore, the secondary or high-speed stage valving is activated at a lower velocity than is desired. Activation of the secondary valving at relatively low velocities creates harshness. Harshness is created because the shape of the fixed orifice bleed circuit force velocity characteristic is totally different than the shape of the high-speed circuit.
Continued development of shock absorbers include the development of a valving system which can provide a smooth transition between a low speed valving circuit and the secondary valving or high speed valving circuit. The smooth transition between these two circuit will help to reduce and/or eliminate any harshness during the transition.
The present invention provides the art with a method for tuning damping forces at low piston velocities in order to improve the handling characteristics of the vehicle without creating harshness. The present invention provides a variable orifice bleed circuit which is incorporated into the secondary valving system. The secondary valving system includes a plurality of discs secured to the piston to close the fluid passages extending through the piston. The plurality of discs deflect due to a pressure differential to open the fluid passages during the second stage valving. The variable orifice bleed circuit of the present invention incorporates a smaller diameter valve disc directly adjacent the valve disc which contacts the piston. This smaller diameter disc allows the outer circumferential portion of the valve disc in contact with the piston to deflect prior to the deflection of the stack of valve discs to provide the variable orifice bleed circuit. In one embodiment, the reduced diameter disc is concentric with the other valve discs, in another embodiment it is eccentric to the other valve discs and in one other embodiment the reduced diameter disc includes a contoured surface to control the deflection of the other valve discs.